What a unit is
A measure of biological activity, not volume, and it is not comparable between products. Dysport units are roughly a third the strength of Botox units, which is why the per-unit price differs and why a Dysport treatment involves more units for the same effect. Any clinic quoting a flat per-area price rather than per-unit is deciding your dose in advance.
Here you are told the units before anything is drawn up, and you are charged for what goes in. If the plan was fifty and forty-two did the job, you pay for forty-two.
Why conservative
Two reasons, one aesthetic and one physiological.
The aesthetic one is obvious: a face with no movement in the upper third reads as treated from across a room, and the people who look best are the ones nobody can tell about. The physiological one is less discussed: the muscles being treated atrophy slightly with sustained use of neuromodulator, so somebody dosed heavily for eight years needs less than somebody dosed lightly, and looks different in ways that were never chosen.
The practical version is that we would rather you returned at four months than looked frozen at two, and that a first treatment here is usually a little lighter than you expected. It is easier to add at the two-week follow-up than to wait out an overdose for three months.
The two-week follow-up
Included, always, and not upselling. Full effect lands at ten to fourteen days, and that is the only honest moment to judge a result. About a third of first treatments get a few more units then; a handful need nothing; nobody is charged for the appointment.